Reinforced ply material commonly comprises an elastomeric (e.g. rubber) sheet and a plurality of reinforcement elements embedded therein. When the ply material is being used to make steel belts for radial tires, the reinforcement elements are usually cabled steel cords which are arranged in a planar row with uniform lateral spacing between the cords. Such ply materials are typically manufactured with an apparatus including a guide insert having passages through which the cabled steel cords pass. Specifically, the guide insert is positioned just upstream of a die throat so that the pattern of its passages dictates the arrangement of the cabled steel cords in the reinforced ply material and thus the steel belt.